Production of metals



g- 19, 1952 i c. H; WINTER, JR 2,607,674

PRODUCTION OF METALS Filed May 25, 1949 IN V EN TOR. (ZzarkyEWrzZm/r ATTOR EY Patented Aug. 19, 1952 2,607,674 7 PRODUCTION OFVMETALSV i Charles H. Winter; Jr., Wilmington, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Delr, a corporation of Delaware V Application May 2 5, 1949, Serial No; 95,215

This invention relates to the preparation of titanium and zirconium metals by reduction of their halides. More particularly it relates to novel methods for preparing such metals in relatively pure, elemental state by reduction of a halide thereof, especially a tetrachloride, with a particular type of metallic reducing agent.

Titanium and zirconium, due to their previous commercial scarcity, have been classed as rare metals. Their unavailability has been due-to the difficulties encountered in prior attempts to separate them from their ores or compounds. Due to their potential importance for use in aviation and other fields, they are now of first-rank importance. V I

- The classical sodium reduction method for producing titanium comprises theheati'ng of titanium tetrachloride and metallic sodium in a steel bomb. The sodium reacts with the chloride to produce sodium chloride and free elemental titanium andthe possibility of contamination of the titanium with oxygen is avoidedin'such process. More recently other metallic reducing agents have been suggested foruse, includingthose of calcium, magnesium, aluminum 'andpotassium. The reducingmetal must in such instances be highly reactive, must not alloy appreciably with the metal being produced, and the chloride byproduct must be capable of ready separation from the metal. Magnesium is especially suited for thislpurpose, since it does not alloy with titanium and is commercially available in large quantities. v Zirconium halides react with magnesium and, the other reducing agents mentioned in somewhat analogous fashion. a

.Several disadvantages attend these prior methods for obtaining metallic titanium or zirconium. Inpreparing titanium, for example, the reduction isgenerally effected at temperatures ranging from 750 1100? C. with formation of the titaniumin sponge form together wane metal halide byproduct "in liquid or molten condition. This spongeliforms uponand tenaciously adheres to the j internal walls of the reactor to eventually grow or bridge across its upper or top portion. Due to the reactivity of titanium at high temperatures with oxygen, hydrogen, nitrogen and water vapor, the sponge must be cooled ;to below the reaction temperature of the titanium before an attemptlto remove and. recoverv such sponge from the reaction vessel can be undertaken. [A considerable amount of. time is thus consumed andprolonged interruptions are encountered in these operations. Consequently,,thesesteps of heating the charge, carrying out of the reaction,

- 6 Claims. (Cl.-7584) 2 I cooling the final product, and discharging it from the reactor, have rendered this prior non-continu- 011s type of operation unattractive and economically disadvantageous for commercially exploiting titanium and zirconium manufacture.

It is among the objects of this inventionto overcome-these and other disadvantages characterizing prior titanium and zirconium metalproducing operations. A primary object is to provide a novel,. commercially useful and continuous type of operation for producing these metals throughreduction of their halides which will be free from the disadvantages of such prior procedures. Additional, specific objects of the invention are to provide a novel, simplified method and apparatus wherein objectionable adherence of the metal sponge reaction product to the walls of the reactor can be readily obviated and from which the sponge can be readily re covered during the 'process without recourse to the periodic shutdowns which prior methods and apparatus have necessarily entailed; and to provide a novel process in which a more accurate, effective control can at' all times be exercised over the reduction operation itself andmore particularly overthe density characteristics of the spongemetal reaction product recovered therefrom. Otherobject's and advantages of the invention will be apparent from the ensuing de- 'scription as well as from' the accompanying drawings in which there is'shown a vertical, sectional view of one useful form of apparatus in which'the invention can be carried out.

These objects are attained in this invention which "broadly comprises reducing in an inert gaseous atmosphere and at an elevated temperature within a reaction zone a halide of titaj- "mum or zirconium with a metallic reducing agent,

and during said reduction suspending said reducing agent upon the surface of a metal salt bath maintained under vor'tical motion within said zone;

In a bra specific and preferred embodiment. the invention comprises reducing titanium tetrachloride with magnesium in an atmosphere of argon and" at temperatures ranging from 850-1000 C. within a reaction zone, throughout said "reduction floating said magnesium on the surface of v a liquid magnesium chloride bath maintained at a constant leveland in vertical motion within said zone, .and during said reduction withdrawing therefrom titanium metal sponge formed therein.

Qj Referring to the drawing, there is shown a is provided in the base of said reactor, through which outlet metal sponge and other reaction products formed within the reactor'can be conveniently discharged, either continuously or semi-continuously, from the reactor. A liquid level control element 5 provided with an overflow pipe or spout member 6, which is in open communication with the interior of the reactor, is :also provided in the base of said reactor, whereby a substantially constant level of :a metal salt bath can be maintained within the reactor and for a purpose to be presently described. Suitably mounted for rotation within the base of the reactorand well below the surface or level of its contained metal salt bath is a conventional blade or paddle type agitator 7. The drive shaft 8 to which its paddle members are secured is driven .at any desired, controlled rate of speed bymeans of a conventionalmotor or other driving' means (not shown). Arranged in concentric,

:spaced relationship about the shaft'8 is an annular casing 9 adaptedto form a'passage l0 having *an inlet .l l, and anoutlet l2. through which-passage'an-inert gas;.such'-as argon, from a-suitable source of supply (not shown) can be fed, as

desired, into the system to maintain an inert atmosphere within the reactor and concurrently preventxleakage of atmospheric 'or'gother objectionable reactivegases intosaid reactor and the system. I r

subjected-to vertical motion by" means ofthe agitator 1.

Thus, "sufficient solid magnesium chloride is first placed within thereactoreand heat applied thereto to induce its melting and liquefaction. Paddle agitator I is then set in motion and at a rate of speed "which is adapted to establish a circulatory motion in the top of the liquid bath fromthe side wallsof the reactor towards the center and thence downwardly'so that-a liquid vortex is formed atsuch center. :At the sametime and throughout the subsequent reducing operation, temperatures rangingfrom about 8QO-900 C. preferably prevail within the reactor which is in closed condition and under an inert atmosphere of argon introduced via conduit -9-and inlet tL- Magnesium -metal in either molten, slug or other finely-divided condition is then fed via inlet 3 onto the surface of the magnesium chloride where it floats upon and collects at the center of. the vortex to quickly melt .under the prevailingitemperatures., Surface movement of the salt bath from'the reactor side walls towards the reactor center causes the, floating, molten magnesium to accumulate' 'onthetop,

centrallportion or the bath. and thus remain out of contact with such reactor walls. At the same time, the axial down-draft of the bath liquid is maintained at a rate which is insufiicient to submerge the pool of molten reducing metal. The reactant metal thus remains at the surface of the bath out of contact with said reactor walls but in constant contact with the titanium tetrachloride being continuously 'introduced'into the reactor via-the inlet 2 and in quantitywhich will provide and maintain a constant atmosphere of that halide over the surface of the molten reducing metal. The magnesium reacts with the titanium tetrachloride to liberate titanium metal and at the same time forms additional quantities of magnesium chloride which, being of greater density than the metal itself, sinks below the 7 surface of the magnesium and joins the supporting magnesium chloride bath to be thus removed from the zone where it might interfere with the intimate contact desired to be effected between the magnesium metal and titanium tetrachloride. As the-magnesium chloride bath increases, it gradually overflows and. is removed from the reactor 'via overflow spout 6. As magnesium metal is consumed during the reaction, additional quantities of that metal can be continuously or intermittently added to the reactor via inlet 3. The titanium metal reaction product can be allowed to collect, by settling through the molten magnesium chloride bath, as finely-divided sponge in the bottom of the reactor and periodically removed therefrom for separation and recovery through discharge outlet 4.- Preferably, however, it is continuously removed from the reactor through said outlet together with a ortion of the liquid magnesium chloride. It can then be subjected to conventional water washing or other form of leaching treatment, or maybe vacuum-distilled to remove magnesium chloride and other undesired impurities to obtain the final, desired pure metal. Thereafter, the metal can be formed into ingotsor briquettes and sub sequently fabricatedtinto the desired titanium metal or alloy.

To a clearer understanding of the invention, the following specific examples aregiven, These are merely illustrative and are not to be construedas'in limitation of the invention:

Eatample I Titanium' metal was produced through reduction of TiCLtWi-th magnesium. An apparatus of the type shown in the accompanying drawing was employed in such reduction. This apparatus consisted of a cylindrical, corrosionresistant steel reaction vessel .24 high, having an internal diameter of 10" and provided with a paddle agitator in its base, the blades of which had an overall length of "8" and were 1%" in Width. This agitator was connected to-a motor driven at R. P. M. During the operation an argon atmosphere prevailed within thereactor, such gas being added thereto at a rateof one cubic foot/hr. During said operation also, the

reactor was maintained at 850 C. by means'jof .an electric heating element in which it was enclosed.

ofits paddles being. soi regulated' as to create a vortexin the center of themolten magnesium chloride. At the sidesyo f the vessel, .jthe' mag'- hesiurn' chloride reached to. a height of 11' from 5 its basez wlrereas atthe centeiit 'reach'e d a height of 7'"? In other w'ordsf the 'vortexwas 4" Heep. Upon 'achieving such'liquid vortex: and motion: 's'lugs'i 6f magnesiumiwre introduced into the-"reactor through "thednlet provided-therefor amrat the rate of one slug pr iminutet'equivalent to 5' 'poun'ds of magnesium metal perfhour); This magnesium collectedf'at" the"centerzfoff the vortex to-bel qificklyimelted-under thehigh' temperature oonditions obtainingwithin 'therreactor. 5 Themoltenmetarfloated'on the magnesium ch10.- ride at .theivortex} and' much asTanTiceberg floats in :water i" i.ie;,i si'nce Lthe specific-:gravity of. mag- -nesiun1" is about 75% that of magnesium chloride, about 5 75% bff'the' metal 'was' zsubmergedi n the salt; bath support." '"rSinu ltaneously;liquid titanium tetrachloride, at the rate of 19.5:pounds per hour; was added'continuously toith'ejreactor, tozbecome: "immediately? vaporized 'upon "encoun 'tefing.the'high"(850 C.)' temperature within-the reactors: '"Upiori' contact at this .TiCh 'v'apor" with themagnesium metal in 'the'argon atmosphere, titanium metal wasiformedwhich:settled to'the .base? of rthe-reactor and was continuously withdrawn; therefrom through its out-lettogether with small-amounts of MgClz liquid; Meanwhile, additional magnesium chloride was-being formed during the reactionandany excess or overflow thereof was'removed by means of the overflow outlet also provided in the base ofthe reactor. The titanium sponge metal thus produced, on

analysis, had the following approximate composi- .tion: m1: e

. recovered sponge was treated under; vac- ;uum distillationto remove the MgCh and other impurities present, to-resultin therecovery-gof pre amet substantially; pure titanium metal from the Mmpl eII E p e- I was duplicated, en a e thattolchiometri pm r 9f magnesium and "liyried zirconium chloride were employed ;in" "the reactionand a temperature of 825 c ex isted in the re'act'or. Fron'r this operation a; pure zirconiummetal-productwas recovered. v

whiledescribed above in its application to'certam-specific and preferred embodiments, the in"- .ventioneis: "not: restricted thereto. Generally,- use isoont'emplated herein of any reducing metal whi'ch is more electropositive thafl' the titanium or zirconium being produced. -Metals"e'specially us'eful for this purpose includ'e tho'sifwhich, in aqueous solutions, Y would hav'e electrode" potentiali-values ofl fld or greater as shownfoythe electfdfiiotiVe forceseries. "Of these, the alkaliand alkaline' ea'rth metals are especially useru1 e- "cause their electrode potentials are "all "greater "than 220,451; fact; which insures rapid reactiofi'at the 150 C. or higher temperature usually-eniployed Ih'er'e'in'. 'Among specific examples of such uSeful "and contemplated T reducing metals are magnesium, calcium; barium,- strontium, aluminurng 'sodium, potassium or lithium. These are molten at 750-'-C. or' higher and have relatively low'specifid 'gravities enabling them to "be floated onthe most suitable metal halide salt baths." 0'! those mentioned, calcium and magnesium are preferred, magnesium being most preferred for use in view of its high potential and production in the reaction of a residue easily separated-from the metal being produced. a v

A variety of metal halidesalts can-be employed as the molten metal bath, provided they are substantially inert with respect to the reactants employed. However, certain restrictions exist as to the" combinations of employable' supportingbath and reducing metal; The relative densities of the reducing metal and the supporting metal salts andf'athe pressures maintained within the reactionf vessel are most critical. The metal must be lessdense than the support; and both must be liquidlwithin thesame temperature and pressure range. 5 Preferably, the bath salt used comprises the same salt as that which is or will be produced in the, reduction reaction. For example, when re ducing TiClI. with magnesium, the magnesium is preferably floated on a melt of magnesium chloride; and when calcium-brother alkaline "earth metals-are I resorted to as reducing agents, a melt of'thelcorresponding' alkaline earth metal h 6, especially a chloride, is preferred fof empl "y ment. similarly sodium may be supported' on sodium chloride, whenjme sodium reduction method isresortedfto'; potassim fori chloride, and calcium ion calcium chlorid ,v Alternatively, magnesiumjnaybe floated o'ri'its bromide salt "and sodium "or' potassium nfthir respective bromide pf iodide saltsi e magnesium nor 'sodiu'nifhowever m "with" their fluorides at srmospn ritprsssfir'e befic'a'us' th'e fluorides are solid at a temperature .wnere th "metals volatilizeirlowever; secures-"- 'tenis ean-be resorted to with addition-lot a low r melting halide salt, seen as sodium or potassiu chlorides; "Oxygen-containing?compounds a're n'ofldesirable"forfuseinthe invention because of their reactivity with titanium r -zirczsnmnrasu the undesired contaminationwhicli would result of the metal 'b'eing" recluded: "Exceptror these restrictions of density and melting te' pera'tiifes', any combination of "reducing ajgiifaind'molt'n liquid metal halide support can be 'gcnerallyem ployed in'theiinventi'on'j The metals 'whicli {thi's invention is-particularly useful for producing 'cb'mprisetitafiiufn {arid zirconium". Any of theirhalides can lie fduced in the process and tl 'iose th' lialogen com onent or which has an atomic number reater "mane, i. chlorine, bromine, or. iodine; are generally 'utilizable. The chloiid esl'siich as I 1C1; and zrcn are .prfer'red" for use since they are"' tlief niost economicalfpractioal a'nd freadil'yTobtaii'ia l6.

fWhile th'se chlorides'are rererred the brqmiaes,

used, as already indicatedIJ-The' 'fiuorid' .are ress desirablefork employment, especially use the alkalineearth fiuoricleswhich result iri' th'e proc ess are insoluble in water an'd difiicult welinii hate in the subsequent purification dperatio'zi. Iodids' andbroinides are also solid'at'robin rent perature' ahd, though emp'lbyablefiare bi-1cm!- cally' disadvantageous 'duet'o their r ati'v'el'y highcostf" The only major difierence betwnt e tit um and zirconium-producing processes that whenusing the' tetrachlorids atfatm'os N' pressures-zirconium tetrachloride a solidsabumes araudug soo fof iddides' 61' fluorides 'of' these jnetalscan arse-be 7 conium tetrachloride "may be difierent.v Aside from this, the involved halides may be added in any desired form, eitherv as solids, liquids, or vapors; 'If added-as liquids, or as solids-they become volatilizedupon 'introductiondnto the heated reactionvessel. p While reactortemperaturesinthe range of from 800-.900 C. and substantially atmospheric pressures are employed during the reaction, such temperatures and pressures, aremerely preferred. If desired; temperatures rangingfrom 750-4100 C.,- as well as pressures above atmosphericland to, say,;from =1-3,at mospheres or higher, can :be resortedto. Again, while argon has been indicated as alpreferred, useful type of'protective or-inert gas foremployment intheinvention, other inert gaseous elements ofigroupOof the periodictable, including helium or;neon, ormixtures' thereof, can be employed. Additionally, any other protective and inert. gas can be used, provided such gas is freefromnndesirable reactants, especial-1y oxygen and nitrogen the presence of which is to beavoided in the system.

As analternative to the continuous or intermittent withdrawal of the-sponge metal reaction product through reactor outlet-mwithdrawal of such sponge can, if desired,. be eifected in the form ofwa spongy ingot by taking advantage of the characteristic of-the titanium-or zirconium reaction :product to adhere'to any solid surface including itself, Thus, this ingot can be started by having present at thebeginning of the reaction a suitable supporting surface or bait material upon-which the sponge product will grow and be subsequently withdrawn through the top ofthe reactor. For example, vasuitable starting material, such as a rod or other form, of titanium, can he suitably, -disposed above 'andinserted through the top :of the; reactor to descend therefrom intothe vortex formed atthe top of the metalhalide bath and contact the pool of molten reducing metal retained in such vortex; I As the reaction proceeds, this rod together with its deposit of'sponge metal-,from the reaction can be slowly withdrawnupwardly from the reactor to proceed through a suitable cooling space and gas-sealed removal lockfor withdrawalto a suitable purification stage of the4process. The ingot will be somewhat porous, and will contain minor amounts of; magnesium chloride and magnesium metal, butthe-latter will continue-to react; with the-titanium or, zirconium halide to which it is exposed within the reactor anduntil it has-passed through the gas locks or other meansadapted to prevent admission of air into the reactor. The, manner of withdrawal of the metal and the physical condition thereof, at. the time of withdrawal-maybe selected at will and may be determined by-the use for which the metal product'is designed; l

A most important feature of the invention is, .as already noted, ithe special motion which-is imparted totthe supporting liquid salt bath. It is due principally to this featureof the invention that thepresent process, is rendered continuous and resort to such liquid motion will befound toobviate many of the difiiculties attendant prior titanium and zirconium metal producing opera tlonsr. Obviously this ,liquidmotion-and maintenance of a vortex-may be achieved in a variety -of,,ways,;the simplest-being by mechanical agitation-and through resort tothe paddle means w i a ea to ril u a has been cr b d-in thefdrawing Various other devices and; medi m mes.t t-W ?wmmnd g e m an c f be resortedv to. For. instance, the reactionvessel equipped, withv a conventional-paddle and. drive shaft may be placed on a. suitable turn-table or other media adapted to rotate said paddle; or the liquid support bath maybe actually pumped out at the bottom of the reactor and returned thereto .in continuous circulation-at thejtopthereof for flow downwardly along-itsisiderwa'lls. This will give the same effect of motion from the side walls to the center or the-vessel and downward. Al-

ternatively, such movementmay be accomplished with" theaid of a submerged, inverted, conical structure so arranged or positioned that there .is a-limited opening at the periphery of ?the vessel. Thesexand other obvious, modificationsto the invention .will be apparent to those skilled'in @"I'he great advantage of this invention and its immediate "value -in j the fieldof titanium and zirconium metal production will be apparent from the fact that for thefirst time it :is possible to obtain-fsuch'metals in a pure state by a continuous operation 'in'which 'the' raw materials may be continuously'added to a reaction vessel and the sponge metal product continuously withdrawn therefrom for recovery and purification.

I claim as'my invention: I l

l. A method for producing a metal selected fromthe group consisting of titanium and zirconium which comprises reducing in an inert atmosphere and at an elevated temperature withina wall-enclosedreactionzone a halide of said metal with a metallic reducing agent'having an electrode potential value of at least 1.70, throughout said reaction floating said reducing agent outof contact with the walls of said reaction zone and on the surface and in the vertex of an inert molten metal salt bath maintained in vortical motion within said zone by mechanical impelling means disposed whollybelow the surface of "said vortex, and withdrawing therefrom the resulting metal reaction product.

2. A method for producing a metal selected from the group consisting of titanium and zirconium which comprises reducing in an inert atmosphere'and at temperatures ranging from 7501100 C. in a wall-enclosed reaction zone, a volatile halide of said metal the halogen component of which has an atomic number greater than 9,with a metallic reducing agent. having an electrode potential value of at least 1;70, which agent, during the reaction and at said temperature, formsa liquid halide, by-product, throughout said reduction-floating said reducing agent out of contact with the walls of said reaction zone on the surface and in the vortex of an inert metal saltbath which is maintained in vorticalmotion withinsaidzone by mechanical impelling means gdisposed wholly below the surface of said vortex. and withdrawingtherefrom the resultingm'etal r c n o Y I 1 a 3. A process for the preparation of-titanium metal which comprises floatingmolten magnesium, onthe surface of a mass of; molten magne- ;siu m chloride maintained Within and partially filling a wall-enclosed reaction zone, maintaining said, zone under an atmosphereof argon and at temperatures rangin -from 800-9 00 C.,-forming a vortex within .said mass'and at .the surface thereof by impartingmotion thereto belowits surface. and from its peripherytowards its center andthence downwardly. but at a rate, which: is insu fiicient to completely submergesaid molten ,rnagnesium in said chloride; but maintains the rnagnesium within; a vortex, and: out :of contac't with the walls of said reaction zone, effecting said vortical motion by mechanical impelling means disposed wholly below the surface of said vortex, maintaining a charge of titanium tetrachloride vapors above the surface of and for reaction with said molten magnesium, and during the reaction withdrawing from said reaction zone titanium metal formed in the reaction.

4. A process for the preparation of zirconium metal which comprises floating molten magnesium on the surface of a mass of molten magnesium chloride maintained within and partially filling a reaction zone, maintaining said zone under an atmosphere of argon and at temperatures ranging from 800-900 0., forming a vortex within said mass and at the surface thereof by imparting motion thereto from its periphery towards its center and thence downwardly but at a rate which is insufficient to completely submerge said molten magnesium in said chloride but maintains the magnesium out of contact with the walls of said reaction zone, effecting said vortical motion by mechanical impelling means disposed wholly below the surface of said vortex, maintaining a charge of zirconium chloride vapors above the surface of and for reaction with said molten magnesium, and during the reaction withdrawing from said reaction zone zirconium metal formed in the reaction.

5. A process for the preparation of titanium metal which comprises floating molten sodium on the surface of a mass of molten sodium chloride maintained within and partially filling a reaction zone, maintaining said zone under an atmosphere of argon and at temperatures ranging from 800-900 0., forming a vortex within said mass and at the surface thereof by imparting motion thereto from its periphery towards its center and thence downwardly but at a rate which is insufiicient to completely submerge said molten sodium in said chloride but maintains the sodium out of contact with the walls of said reaction zone, effecting said vortical motion by mechanical impelling means disposed wholly below the surface of said vortex, maintaining a charge of titanium tetrachloride vapors above the surface of and for reaction with said molten- 10 sodium. and during the reaction withdrawing from said reaction zone titanium metal formed in the reaction.

6. A process for the preparation of titanium metal which comprises floating molten calcium on the surface of a mass of molten calcium chloride maintained within and partially filling a reaction zone, maintaining said zone under an atmosphere of argon and at temperatures ranging from SOD-900 C., forming a vortex within said mass and at the surface thereof by imparting motion thereto from its periphery towards its center and thence downwardly but at a rate which is insufficient to completely submerge said molten calcium in said chloride but maintains the calcium out of contact with the walls of said reaction zone, efiecting said vortical motion by mechanical impelling means disposed wholly below the surface of said vortex, maintaining a charge of titanium tetrachloride vapors above the surface of and for reaction with said molten calcium, and during the reaction withdrawing from said reaction zone titanium metal formed in the reaction.

CHARLES H. WINTER, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,437,984 Marden Dec. 5, 1922 2,129,445 Rehns Sept. 6, 1938 2,148,345 Freudenberg Feb. 21, 1939' 2,171,439 Von Zeppelin Aug. 29, 1939 2,205,854 Kroll June 25, 1940 2,214,211 Von Zeppelin et al. Sept. 10, 1940 2,482,127 Schlechten et al. Sept. 20, 1949 2,556,763 Maddex June 12, 1951 2,564,337 Maddex Aug. 14, 1951 OTHER REFERENCES The Electrochemical Society, Preprint 78-11, October 7, 1940. Pages 161 and 163.

The Electrochemical Society, Preprint 88-30, October 17, 1945. Pages 357 and 358. 

1. A METHOD FOR PRODUCING A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM AND ZIRCONIUM WHICH COMPRISES REDUCING IN AN INERT ATOMSPHERE AND AT AN ELEVATED TEMPERATURE WITHIN A WALL-ENCLOSED REACTION ZONE A HALIDE OF SAID METAL WITH A METALLIC REDUCING AGENT HAVING AN ELECTRODE POTENTIAL VALVE OF AT LEAST 1.70 THROUGHOUT SAID REACTION FLOATING SAID REDUCING AGENT OUT OF CONTACT WITH THE WALLS OF SAID REACTION ZONE AND ON THE SURFACE AND IN THE VORTEX OF AN INERT MOLTEN METAL SALT BATH MAINTAINED IN VORTICAL MOTION WITHIN SAID ZONE BY MECHANICAL IMPELLING MEANS DISPOSED WHOLLY BELOW THE 